What was the 'threadlike' thing which the inventor grasped? Why was it so frightening?

It was some string-like creature that was behind him. The TT grabs it and rips part of it off but it is still alive looking at him:

I turned, and I saw that I had grasped the antenna of another monster crab that stood just behind me. Its evil eyes were wriggling on their stalks, its mouth was all alive with appetite....